@Andrew Wooster is 95% correct....
The Phone Numbers->DID/DNIS tab is for incoming DNIS Routing and has no affect, or relation to ANI.
You can specify an outbound ANI on the User object (which, coincidentally has no effect on inbound DNIS routing).
Outboud ANI can be set on the User, The Station, The Workgroup, The Campaign (in the case of Dialer) or the Line. The Line is the "default" or "last resort" value that is used, if it isn't set anywhere else.
@Mirza Baig If I'm reading your requirement correctly, the agent calls in to an IVR, is then prompted for an outbound number and you call that number, but want to present the user's ANI? (So, no client?)
This could be done, but will probably require Handler modifications.....